<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
  <style>
    .parent{
      position: fixed;
      top: 20px;
      left: 20px;
      width: 100px;
      height: 100px;
      border: 1px solid #ccc;
    }

    .child {
      position: absolute;
      background-color: red;
      width: 20px;
      height: 20px;
      left:-20px;
      bottom:0
    }
  </style>
</head>
<body>
  <div class="parent">
      <div class="child"></div>
  </div>
</body>
</html>